Capital Projects:

An interim report into the failure of nine heavy-duty nuts on the Forth Road Bridge has concluded that all 1,888 similar nuts on the bridge should be replaced.

Bridge authority FETA ordered the investigation after bridge inspectors discovered nine cracked nuts during a routine inspection in 2008. Consulting engineers FaberMaunsell (now Aecom) were appointed to carry out laboratory testing and a desk study of similar details on suspension bridges throughout the world.

The nuts are used to secure 192 “cable bands” to the bridge’s main cables – metal castings over which the steel rope hangers holding up the roadway are looped. Each cable band is held in place by a number of 35 mm diameter high tensile steel bolts, pre-tensioned to a load of around 80 tonnes

All the cable band nuts and bolts were replaced in the late 1990s as part of a larger project to replace the hanger ropes. The bridge authority’s investigation has identified a number of design and specification decisions and construction methods that may have contributed to the cracking:

• The replacement of the original 1 ½ inch bolts with metric M39 bolts resulted in the nuts having a thinner section than the originals
• The dimensions of the new nuts are particularly small compared with similar nuts on other suspension bridges
• The much higher grade of steel specified means the nuts are harder and less flexible than the originals
• Misalignment of washers may have led to uneven loading in the nuts
• The protective coating on the nuts was inadequate and allowed moisture to cause damage

Four of the failed nuts were replaced on the west cable, using access platforms already in place. The five failed nuts on the east cable were due to be replaced during 2009.

The investigation's final report was subsequently delayed partly due to the discovery of another cracked nut in June 2009, bringing the total found to ten.

Further laboratory testing is under way, which will be complete by the end of October 2009.  It is now expected that a report will be brought to the December 2009 Board meeting.